1. Field of the Invention
This invention relates to a method of upgrading existing Global Navigation Satellite System (GNSS) user equipment, such as a GPS receiver, in order to add high-performance, tightly integrated navigation and communication (Nav-Com) capability without the need to modify the existing equipment. The invention also relates to an apparatus, which may take the form of a plug-in enhancement module, for adding iGPS to existing GNSS user equipment.
In a preferred embodiment of the invention, the upgrade is to a particular tightly integrated Nav-Com system known as iGPS, which utilizes the carrier phase of signals received from Low Earth Orbiting (LEO) satellites, such as Iridium to provide a special-purpose wideband reference signal. In this embodiment, upgrade to iGPS is achieved by:                using an existing antenna port in the GNSS user equipment to supply the GNSS user equipment with a special-purpose wideband reference signal phase locked to the reference oscillator of the apparatus;        causing the existing GNSS user equipment to produce coherent correlations of incoming GNSS (or GPS) signals and the reference signal relative to the GNSS user equipment reference oscillator; and        sending the coherent correlations back through an existing data port to a Nav-Com processor for combination with additional correlations of Iridium taken relative to the reference oscillator of the apparatus to derive more precise solutions for position, velocity, and/or time.        
The GNSS user equipment may be a Defense Advanced GPS Receiver (DAGR), with the Nav-Com processor and special purpose reference signal generating components being provided in a single unitary module that plugs into existing ports of the DAGR, without need to modify the DAGR. This arrangement can significantly improve the accuracy, integrity, and availability of Position, Navigation, and Timing (PNT) in the DAGR, in some cases by three orders of magnitude, using carrier phase with the potential to converge onto sub-decimeter level position fixes with time frames on the order of a minute anywhere on the globe. The invention enables the high precision GNSS carrier phase observable to be more readily exploited to improve PNT availability—even under interference conditions or occluded environments. Furthermore, the invention enables new communication enhancements made available by the synthesis of precisely coupled navigation and communication modes. With the proper design and integration, the easily attached upgrade is capable of significantly lowering fielding and life cycle costs to realize the advanced capability.
2. Description of Related Art
Tightly integrated Navigation and Communication opens up a vast realm of new complementary capability for U.S. military, civil, and commercial applications, especially if such infrastructure is global in nature. Communications infrastructure can improve navigation by providing real-time data and timing aiding, while navigation infrastructure improves communication by providing time and position aiding. Employing a global infrastructure enables stakeholders to better enjoy economies of scope and scale. The more tightly integrated the architecture of the Navigation and Communication components, the greater the mutual synergies can be achieved.
One especially notable example of an integrated global Nav-Com system is iGPS, created by the fusion of the Iridium and GPS global satellite constellations. The Navy has awarded a contract to a Boeing-led team to use Iridium to provide supplemental data, timing, and ranging information to authorized GPS users. This additional information provides among other benefits the means for significantly improved interference rejection performance and faster acquisition of GPS with a dynamic user platform [Glen Gibbons, “Boeing Wins NRL Contract to Continue Iridium/GPS Development”, Inside GNSS, September/October 2008].
In general iGPS, as well as the broader global Nav-Com solution set, has the potential to significantly improve the accuracy, integrity, and availability of Position, Navigation, and Timing (PNT). The rapid angle motion of LEO satellites in the sky dramatically increases spatial diversity over the traditionally slow moving GPS satellites in high orbit. If the LEO and GPS satellite carrier phase is employed, there is potential to lock onto sub-decimeter level position fixes with time frames on the order of a minute anywhere on the globe. See, for example, U.S. Pat. Nos. 6,373,432, 5,812,961, and 5,944,770.
The use of carrier phase provides significant dividends for users. The GPS L1 wavelength is approximately 19 cm. The intrinsic precision for GPS is a small fraction of this wavelength. The typical timing and ranging error budget for GPS L1 works out to be on the order of 0.5 cm RMS or 20 ps in terms of time. This precision is the key to achieving the overall position accuracy just mentioned as well as integrity and interference rejection. The iGPS infrastructure can be used to provide both data aiding (for Iridium ephemeris and GPS data stripping) as well as time stability transfer (calibrating the Iridium clock with a reference station and broadcasting precise Iridium carrier phase corrections to the user in real time), as described in U.S. Pat. No. 7,372,400.
The converse is also true. Once the user position and time are well known, new capabilities related to improved communication are possible. For example, a carrier based upon an ultra-stable virtual clock can be established between the user and a satellite because the user has full knowledge of the position and timing of each. This enables robust coherent communication links to be established to support, for example, interference resistant and low probability of intercept communications.
Traditionally, carrier phase has not been exploited by the military for navigation purposes. Instead, signal squaring techniques are employed, which have the unfortunate effect of squaring both the signal and the noise. The result has been wasted GPS signal power at a time when the military is considering development and launch of higher-power satellites to make up the shortfall. iGPS infrastructure enables more efficient use of existing GPS power.
Additional global integrated Nav-Com benefits result from further synergies. With GPS user equipment and other devices there is often a need to securely disseminate encryption keys. Without a suitable infrastructure, the process can become cumbersome. For example, with only a one-way data link, users and devices may not be able to authenticate with the key management authority. A robust, global, two-way communication system solves this problem by enabling the user and device to authenticate each request to re-key no matter where they are in the world. This ease of use enables key dissemination to be both secure and effortless.
In the case of iGPS, the U.S. has an opportunity to rapidly implement an existence proof of a LEO-based enhancement to GPS. The Iridium satellites are already on orbit with a lifetime projected to extend beyond 2014 (see “Iridium Satellite LLC Estimates Constellation Life Span To Extend Well Beyond Original Predictions,” Iridium Satellite LLC Press Release, Feb. 26, 2003). Under the above-listed Navy contract, the Boeing team will develop global ground infrastructure and develop new flight software for the existing Iridium satellite constellation by the beginning of 2011. This timetable will provide several years of a suitable signal in space for the U.S. Military and other authorized users to make effective use of the new capability before the Iridium constellation degrades beyond its useful life.
However, a significant obstacle to implementing iGPS is the cost and effort to outfit user equipment such as the Defense Advanced GPS Receiver (DAGR), which remains user equipment of choice with the military. The U.S. Military has currently fielded several hundred thousand GPS units (“Rockwell Collins delivers 200,000th DAGR and 40,000th GPS engine to the U.S. Army”, Rockwell Collins Press Release, Apr. 18, 2008) and many more are already in the process of procurement. The U.S. Government has purchased these units for nearly $2,000 per unit.
If the Military or other users are to adopt iGPS, there needs to be a straightforward way to take advantage of the installed base of user equipment. Prior art has so far presented two unpalatable approaches: (i) modify the existing user equipment hardware to accept a new precision iGPS interface capable of tight integration and (ii) completely replace the existing user equipment with new tightly integrated iGPS user equipment designed from scratch.
The first approach has caused significant concern because of the economic and technical risk associated with introducing a precision iGPS interface with tight integration. In particular, since the DAGR does not provide for an external oscillator input, one would have to be added. It is not clear how much this changeout would cost and to what extent it would require replacement of DAGR components. There is also a related logistical and configuration control issue that having multiple versions of DAGR hardware would become cumbersome to manage for the users and leadership because, when hardware modifications are made, many of the overall specifications will need to change and be managed. In addition, there is also technical risk associated with the hardware modifications. The carrier phase precision of iGPS for tight integration requires 20 ps stability between the GPS and Iridium signal processing components over the full range of environmental conditions. The hardware components that are especially subject to phase variation include the GPS RF front end, the Iridium RF front end, and the GPS oscillator. While the navigation processing algorithms can tolerate a slow drift of carrier phase bias between the two components, if thermal or mechanical disturbances are excessive, the system will be incapable of providing useful performance. The DAGR layout compounds the technical risk because the Iridium and GPS components are by necessity in different boxes which will be subject to different thermal and mechanical stress.
The second approach also encounters resistance. Given that the U.S. has already made a significant investment in GPS equipment, it is difficult to justify displacing existing inventory.
What is needed is a means for demonstrating the far-reaching benefits of a LEO-enhanced GPS Nav-Com system to its potential U.S. military, civil, and commercial stakeholders. To this end what is needed is an existence proof in the form of iGPS formed by integrating Iridium and GPS wherein there is a practical and attractive method to upgrade user equipment for existing users of GPS. In other words, what is therefore needed is a practical method for creating a tightly integrated global Nav-Com upgrade to an existing DAGR that provides the full necessary precision and performance without need for any hardware modifications.